Bacterial populations undergo a wide range of coordinated changes via a density-controlled process termed quorum sensing. A recently-discovered interspecies quorum-sensing pathway controls critical metabolic pathways in many bacteria and is therefore a promising target for antimicrobial therapies. This pathway is mediated by autoinducer-2 (AI2). AI2 binds to the periplasmic receptor protein LuxP triggering a dephosphorylation cascade through the sensor kinase LuxQ that stimulates cell-density-dependent transcription. The overall goal of this proposal is to elucidate the regulation of AI2 signal transduction by describing the interaction of LuxP and LuxQ in mechanistic detail. The specific aims of this proposal are to: (1) solve the crystal structures of LuxP-LuxQ protein complexes to elucidate the regulation of LuxQ signaling by LuxP, (2) use biochemical and Vibrio harveyi bioluminescence assays to determine the significance of specific LuxP-LuxQ interactions observed in the protein complex crystal structure, and (3) perform biochemical and structural studies to determine the mechanism of action of AI2 analogs in order to iteratively improve the analogs as well as elucidate the contributions of AI2 ligand groups to the binding energy.